Last week, the American Society for Microbiology posted a story that caught my eye which highlighted the most recent work of Kim Lewis and his collaborators published in Chemistry and Biology in March (see citation below). It caught my eye due to the term “siderophores” in the title. You may be wondering why a strange Greek term like that would be of any interest to a soil microbiologist like me, so let me share my fascination for these weird and wonderful molecules and how they may be changing approaches in microbiology.

First, let’s define the word siderophore in simple language.

It’s from the original Greek meaning “iron carrier” and describes a small molecule specifically designed to address the biological iron requirement. While you and I might be able to eat our Flintstones and therefore don’t need to worry about iron, the vast majority of life on earth has to figure out a way around the fact that biological systems require the soluble Fe2+ (ferrous iron), but iron for the most part exists in the environment as the highly insoluble Fe3+ (ferric iron). Why? Because life evolved when the earth was anaerobic (meaning there was virtually no oxygen); there was an overabundance of iron all over the place in the ferrous form. Gradually life took over the face of the planet, turning out atmosphere aerobic, and all that oxygen essentially ‘spoiled’ the iron-rich crust of the earth, oxidizing the iron into the ferric form, which is essentially insoluble and not bioavailable.

Long-story short, we’ve all developed some strategy to deal with this problem. Humans use transferrin in our bloodstream to bind iron, while bacteria, fungi, and some plants use what we call siderophores. These small molecules grab onto ferric iron in the environment and chelate it, forming soluble iron complexes, making it available for biological purposes. For a really gre,at in-depth review of siderophores, look here.

Curiouser and curiouser…

What has intrigued me for years about siderophores is a uniquely discriminating selectivity. I know that may sound redundant, but what I’m trying to say is that certain microbial siderophores are extremely specific to iron and actually can scavenge iron from metamorphic rocks, oxides, hydroxides, you name it they can leach it out, to the point that they even have biotechnological applications in mining activities. At the same time, other microbial siderophores may be able to pick up any positively charged metal or micronutrient in the environment. This includes all the common heavy-metal contaminants that I deal with on a daily basis, from lead and cadmium, to arsenic and uranium. Certain siderophores can chelate these toxic metals and bind them in such a way as to allow them to precipitate out of solution, making them no longer a toxic threat to the organism. Even more fascinating is that the same siderophore, under different conditions, might bind a scarce essential nutrient so the microbe can take it up and continue living normally. There has also been evidence recently to support that microbial siderophores may be involved in quorum sensing (microbial cross-talk) and biofilm formation, but the new work by Kim Lewis and his colleagues brings it all to a new level.

The “great plate county anomaly”

For years, microbiologists have been working on new ways to culture (or, grow in the lab) environmental microorganisms and attempts to overcome the “great plate count anomaly” have included a multitude of creative approaches. All my microbiologist-readers will be intimately familiar with that little problem, but for those of you out there who aren’t … the “great plate count anomaly” describes the fact that no matter how many microbes we can see under a microscrope or detect DNA for in any given sample (soil, marine sediment, water, mucosa, etc), we can only successfully get around 0.1 – 1% to grow on Petri plates in the lab. This severely limits what we can find out about how these organisms function out in the real world and has stalled many aspects of microbial ecology.

The authors were able to definitively prove the siderophores produced by some “helper” microbes actually allowed the growth of other, previously uncultured, bacteria on Petri plates in the lab. They were able to identify cooperative pairs of organisms, in which one organism was identified as the helper and secreted siderophores into the media, and the other organism was only capable of growth when exposed to the siderophores of the helper. They tried an array of synthetic (store-bought) siderophores, which worked in some cases but others did not. They also tried to supplement with high quantities of bioavailable iron, which also, worked in some cases (allowed the growth of previously uncultured microorganisms), and in others did not. This means the helpers likely have very specific relationships with the organisms that rely on their siderophores, with a great degree of discrimination, despite the fact the pairs of organisms were not closely related, not even in the same genus. It may also mean that the function of the siderophore in the media may not have anything to do with iron in some cases. By using these helper organisms the authors were actually able to culture as much as 40% of the total community, which is a vast improvement of the typical <1% we see from most environmental samples.

As a scientist who has literally watched this phenomena unfold on my own Petri plates, but had no clear explanation at the time, I am truly enthralled by this new discovery. It raises some very interesting questions with regard to the microbial ecology of this helper-pair system, in addition to opening doors with regard to antimicrobial therapies and basic culture techniques.

But, I have to wonder, why? What’s the purpose of this highly complex and specific signaling and nutrient acquisition cross-talk? How would it benefit the microbial community as a whole? Is this cooperation and microbial co-evolution at its best, right before our eyes? Things I’ll be pondering over the weekend…